A syntrophic two-member consortium consisting of Geobacter metallireducens and Methanosaeta harundinacea that performs direct interspecies electron transfer (DIET) during ethanol oxidation. G. metallireducens oxidizes ethanol and transfers electrons directly to M. harundinacea via electrically conductive pili and aggregates, bypassing the need for diffusible electron carriers like H2 or formate. M. harundinacea uses these electrons to reduce CO2 to methane. This coculture forms conductive aggregates and achieves stoichiometric conversion of ethanol to methane via DIET. Unlike hydrogen-mediated syntrophy, DIET relies on biological electrical connections between cells, with conductive aggregates serving as conduits for long-range electron transfer. M. harundinacea is an obligate acetoclastic methanogen that can also accept electrons via DIET for CO2 reduction, making it unique among acetoclastic methanogens in supporting direct electron transfer mechanisms.
Taxonomy
| Taxon | Ontology ID | Functional Roles | Abundance |
|---|---|---|---|
| Geobacter metallireducens | NCBITaxon:28232 · TaxonMech · TraitMech |
PRIMARY_DEGRADER
SYNTROPHIC_PARTNER
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N/A |
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| Methanosaeta harundinacea — ontology label: Methanothrix harundinacea | NCBITaxon:301375 · TaxonMech · TraitMech |
SYNTROPHIC_PARTNER
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N/A |
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Ecological Interactions
The graph shows only explicitly linked interaction participants. Community members without an asserted participant link are omitted; membership alone does not imply an interaction.
Geobacter Ethanol Oxidation
In the defined Geobacter metallireducens-Methanosaeta harundinacea coculture, Geobacter oxidizes ethanol to acetate and releases electrons for DIET. This is the donor reaction, not a claim that ethanol carbon is directly oxidized to CO2 at this step. The acetate coproduct also contributes to bulk methane through a separate route, not yet represented as its own graph branch.
Source Taxon: Geobacter metallireducens
Metabolites: ethanol (CHEBI:16236 · MediaIngredientMech · CMMMech), acetate (CHEBI:30089 · MediaIngredientMech · CMMMech)
Biological Processes:
- ethanol catabolic process (GO:0006068 · PathwayMech)
Ethanol oxidation releases reducing equivalents that Geobacter supplies to the defined partner through DIET. This arrow routes electron supply, not acetate carbon, and does not assert that ethanol oxidation selectively causes aggregate formation.
Evidence
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doi:10.1039/C3EE42189A - SUPPORT (IN_VITRO)"stoichiometrically converted ethanol to methane"
DIET in Defined Coculture Aggregates
SYNTROPHYDefined G. metallireducens-M. harundinacea cocultures form mixed aggregates and support DIET; the pilA-deficient Geobacter comparison supports a requirement for its pili-associated machinery under these conditions. This evidence is distinct from Geobacter-species and Methanosaeta concilii expression in brewery-digester aggregates. Aggregate formation itself was not selectively perturbed, and the methanogen electron-entry components remain unresolved. Do not transfer measured conductivity of environmental aggregates to these defined aggregates as a separately measured property.
Source Taxon: Geobacter metallireducens
Target Taxon: Methanosaeta harundinacea
Biological Processes:
- electron transport chain (GO:0022900 · PathwayMech)
DIET supplies electrons for M. harundinacea CO2 reduction to methane, supported by the defined-coculture radiotracer and pilA comparison. This direction does not allocate acetate-derived methane to the CO2 branch or establish a particular methanogen entry protein.
Evidence
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doi:10.1039/C3EE42189A - SUPPORT (IN_VITRO)"Transcriptomic, radiotracer, and genetic analysis demonstrated that M. harundinacea accepted electrons via DIET"
DIET-Linked Carbon Dioxide Reduction to Methane
M. harundinacea accepts DIET electrons for CO2 reduction to methane in the defined coculture, supported by transcript, radiotracer and genetic analyses. Bulk methane also includes acetate conversion. The reported stoichiometry and bicarbonate radiolabel are consistent with roughly one-third of methane arising through CO2 reduction, not all methane. This node represents that CO2 route; it does not replace the missing acetate branch or identify the methanogen surface electron receptor.
Source Taxon: Methanosaeta harundinacea
Metabolites: methane (CHEBI:16183 · MediaIngredientMech · CMMMech), carbon dioxide (CHEBI:16526 · MediaIngredientMech · CMMMech)
Biological Processes:
- methanogenesis (GO:0015948 · PathwayMech)
Evidence
-
doi:10.1039/C3EE42189A - SUPPORT (IN_VITRO)"reduction of carbon dioxide to methane"
Knowledge gaps & discussions (3)
KNOWLEDGE_GAP RESOLVED Alongside DIET-driven CO2 reduction, does M. harundinacea also cross-feed on the acetate generated by G. metallireducens ethanol oxidation, and what fraction of methane comes from each route in this defined coculture?
The earlier discussion deferred an acetate branch because only the abstract was accessible and secondary attributions were insufficient. The primary full paper is now available and has been read; its defined-coculture evidence resolves the biological/source-access question, while graph implementation remains a separate open curation task.
Attaches to: ecological_interactions#Geobacter Ethanol Oxidation, ecological_interactions#DIET-Linked Carbon Dioxide Reduction to Methane
1 evidence item(s)
Primary defined-coculture Results and Figures 4c/5, reviewed directly rather than inferred from the environmental expression excerpt or secondary reviews.
Resolution: Primary Figure 4c separates acetate conversion from DIET-dependent CO2 reduction, and Figure 5 supports the route distinction by bicarbonate radiolabel. See the source hash and inspection scope in the batch46 decision ledger. The missing branch remains open under #1482; resolving this discussion does not mark the graph complete.
KNOWLEDGE_GAP OPEN Which methanogen surface components receive DIET electrons?
The positive DIET result does not identify the receiver proteins or selectively establish aggregate formation itself as necessary. Targeted perturbations would distinguish these mechanisms. Canonical Methanosaeta/Methanothrix naming is preserved; stronger non-graph aggregate and uniqueness wording is not certified by this graph-only review.
Attaches to: ecological_interactions#Geobacter Ethanol Oxidation, ecological_interactions#DIET in Defined Coculture Aggregates, ecological_interactions#DIET-Linked Carbon Dioxide Reduction to Methane
CURATION_TODO OPEN Complete the separately supported acetate branch after Edison review.
The primary evidence is now available, but an acetate-transfer/conversion branch is new causal structure. A one-community Edison dry run completed without API calls or cost; specific credit authorization and report inspection remain pending under #1482. Do not count the bounded CO2 route as completion of the whole graph.
Attaches to: ecological_interactions#Geobacter Ethanol Oxidation, ecological_interactions#DIET in Defined Coculture Aggregates, ecological_interactions#DIET-Linked Carbon Dioxide Reduction to Methane
Environmental Factors
| Factor | Value | Unit |
|---|---|---|
| Anaerobic Conditions | Strict anaerobic | N/A |
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| Conductive Aggregate Formation | Required for DIET | N/A |
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| Pili Expression | Essential for DIET initiation | N/A |
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Growth Media
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